This invention relates to a method for recovering an oil residue from an oil-containing clay and treating the oil-containing clay without causing the environmental contamination.
In the decoloration of an animal and vegetable oil and fat using clay or mixtures of clay and active carbon as a decoloring agent, coloring matters present in the oil and fat are removed by adsorption. At this time, 10-50% of the oil and fat based on the weight of the decoloring agent are adsorbed or attached and therefore, the clay containing the oil and fat as well as the coloring matters is obtained.
The term of an "oil-containing clay" used hereinafter means a mixture consisting mainly of clay containing an animal and vegetable oil and fat, which is by-produced in the decoloration step of the oil and fat.
Because the oil-containing clay can not be disposed or discarded as it is, there has, hitherto, been effected a treatment method including steps of adding a small amount of alkali and water into the oil-containing clay, boiling it for 2-3 hours, allowing same to stand for 6-24 hours, separating and recovering the resulting oil portion and thereafter, discarding clay sludges or utilizing them for land filling.
With this conventional method, however, the recovery percentage of oil is low so that about 10% of oil are still maintained in the clay sludge after the recovery of oil. Therefore there are disadvantages that the environmental contamination is caused by discarding the clay sludge, the treatment efficiency is lowered due to the prolonged time for separating the oil portion, and the waste water discharged after the recovery of oil contaminates water because it has a high COD of 15,000-30,000 ppm and a high oil content of more than 1,000 ppm.